Multiple polynomial digital predistortion

ABSTRACT

A system is provided for pre-distorting a transmit signal in a mobile terminal prior to amplification by a power amplifier to compensate for AM to AM and AM to PM distortion over an input value range. Pre-distortion circuitry includes both amplitude and phase pre-distortion circuitries. The amplitude pre-distortion circuitry distorts an amplitude component of a polar transmit signal using an amplitude compensation signal that essentially cancels the AM to AM distortion, and the phase pre-distortion circuitry distorts a phase component of the polar transmit signal using a phase compensation signal that essentially cancels the AM to PM distortion. The amplitude and phase compensation signals are generated based on corresponding sets of coefficients selected from a number of sets of coefficients defining polynomials describing compensation signals for each of at least two subsets of the input value range for each of two or more power levels of the power amplifier circuitry.

This application is a Divisional of U.S. utility patent application Ser.No. 10/874,509, filed Jun. 23, 2004, currently pending, the disclosureof which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to pre-distortion of a transmit signal tocompensate for amplitude and phase distortion characteristics of a poweramplifier.

BACKGROUND OF THE INVENTION

Transmitters form a significant portion of most communication circuits.As such, they assume a position of prominence in design concerns. Withthe proliferation of mobile terminals, transmitter design has progressedin leaps and bounds as designers try to minimize components and reducesize, power consumption, and the like. Likewise, modulation schemes arecontinuously updated to reflect new approaches to maximize informationtransfers within limited bandwidth constraints. Changes in standards orstandards based on newly available spectrum may also cause designers toapproach the design of transmission systems with different modulationtechniques.

Many different standards and modulation schemes exist, but one of themost prevalently used in the world of mobile terminals is the GlobalSystem for Mobile Communications (GSM). GSM comes in many flavors, notthe least of which is General Packet Radio Service (GPRS). GPRS is a newnon-voice value-added service that allows information to be sent andreceived across a mobile telephone network. It supplements today'sCircuit Switched Data and Short Message Service. GSM allows manydifferent types of mobile terminals, such as cellular phones, pagers,wireless modem adapted laptops, and the like, to communicate wirelesslythrough the Public Land Mobile Network (PLMN) to the Public SwitchedTelephone Network (PSTN).

One relatively recent change has been the advent of the Enhanced Datafor GSM Evolution (EDGE) scheme in GSM systems. This system containsamplitude modulation components, and, as a result, the power amplifiermust be linear and should not operate in saturation when classicalmodulation techniques are employed. Such a linear system lacks theefficiency of one that operates the power amplifier in saturation.

If a polar modulation system is used instead of a classical modulationsystem, then the power amplifier may operate in saturation andefficiency would be greatly improved. In addition, if the polar signalsare generated by a digital method, such a system does not require theuse of a high current drain quadrature modulator. Quadrature modulatorsare undesirable from a design standpoint in that they draw large amountsof current, and hence, drain batteries comparatively fast.

Unfortunately, further complicating matters, the amplitude signal thatcontrols the power amplifier will cause unwanted phase components to becreated in the output of the power amplifier due to the non-linearitiesof the power amplifier. This is sometimes called amplitude to phase (AMto PM) distortion, and it degrades the spectral purity of the system andthe Error Vector Magnitude. Thus, a need also exists to be able tocounteract or eliminate the unwanted AM to PM distortion of thetransmitted phase signal.

An additional concern is that the power amplifier may have a non-lineargain with varying output power. This may create what is called amplitudeto amplitude (AM to AM) distortion. The AM to AM distortion may haveboth phase and amplitude distortion components, and to create a bettercontrol system, these should be reduced or eliminated as well.

One solution used to counteract AM to AM and AM to PM distortion is tomeasure the AM to AM and AM to PM distortion of the power amplifier, andthen create a polynomial for amplitude pre-distortion and a polynomialfor phase pre-distortion that are used to distort a signal prior toamplification by the power amplifier. This pre-distortion offsets the AMto AM and AM to PM distortion of the power amplifier. However, thepolynomials are often at least third order polynomials and are costly toimplement. Further, the polynomials may not effectively offset the AM toAM and AM to PM distortion of the power amplifier at every power levelof the power amplifier. Thus, there remains a need for a more costeffective system for correcting AM to AM and AM to PM distortion of apower amplifier at every power level of the power amplifier.

SUMMARY OF THE INVENTION

The present invention provides a system and method for pre-distorting atransmit signal in a mobile terminal prior to amplification by a poweramplifier to compensate for AM to AM and AM to PM distortion caused bythe power amplifier. In general, the system includes pre-distortioncircuitry having both amplitude pre-distortion circuitry and phasepre-distortion circuitry. The amplitude pre-distortion circuitrydistorts an amplitude component of a polar transmit signal using anamplitude compensation signal that essentially cancels the AM to AMdistortion of the power amplifier. The phase pre-distortion circuitrydistorts a phase component of the polar transmit signal using a phasecompensation signal that essentially cancels the AM to PM distortion ofthe power amplifier.

The amplitude pre-distortion circuitry generates the amplitudecompensation signal based on a first set of coefficients selected from afirst number of sets of coefficients. Each set of coefficients defines apolynomial describing the amplitude compensation signal for one of atleast two subsets of an input value range of the power amplifiercircuitry for one of at least two power levels of the power amplifiercircuitry. Based on a transmit power control signal, a group of thefirst number of sets of coefficients are selected. The group of sets ofcoefficients define the polynomials describing the amplitudecompensation signal for the subsets of the input value range for a powerlevel indicated by the transmit power control signal. The first set ofcoefficients are selected from the group of the first number ofcoefficients based on the amplitude component of the polar transmitsignal, where the amplitude component is indicative of the subset of theinput value range.

The phase pre-distortion circuitry generates the phase compensationsignal based on a second set of coefficients selected from a secondnumber of sets of coefficients. Each of these sets of coefficientsdefines a polynomial describing the phase compensation signal for one ofthe at least two subsets of the input value range for one of the two ormore power levels of the power amplifier circuitry. Based on thetransmit power control signal, a group of the second number of sets ofcoefficients are selected. The group of sets of coefficients define thepolynomials describing the phase compensation signal for the subsets ofthe input value range for the power level indicated by the transmitpower control signal. The second set of coefficients are selected fromthe group of the second number of coefficients based on the amplitudecomponent of the polar transmit signal, where the amplitude component isindicative of the subset of the input value range.

Those skilled in the art will appreciate the scope of the presentinvention and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 illustrates a mobile terminal including pre-distortion circuitryaccording to one embodiment of the present invention;

FIG. 2 illustrates AM to AM and AM to PM distortion of a typical poweramplifier;

FIG. 3A is a detailed block diagram of the pre-distortion circuitryaccording to one embodiment of the present invention; and

FIG. 3B is a detailed block diagram of the pre-distortion circuitryaccording to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the invention and illustratethe best mode of practicing the invention. Upon reading the followingdescription in light of the accompanying drawing figures, those skilledin the art will understand the concepts of the invention and willrecognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

While the present invention is particularly well-suited for use in amobile terminal, and particularly a mobile terminal that is operating inan Enhanced Data for GSM Evolution (EDGE) scheme in a GSM system, itshould be appreciated that the present invention may be used in othertransmitters, either wireless or wirebased, as needed or desired.

The present invention is preferably incorporated in a mobile terminal10, such as a mobile telephone, personal digital assistant, or the like.The basic architecture of a mobile terminal 10 is represented in FIG. 1,and may include a receiver front end 12, a radio frequency transmittersection 14, an antenna 16, a duplexer or switch 18, a baseband processor20, a control system 22, memory 24, a frequency synthesizer 26, and aninterface 28. The receiver front end 12 receives information bearingradio frequency signals from one or more remote transmitters provided bya base station (not shown). A low noise amplifier 30 amplifies thesignal. A filter circuit 32 minimizes broadband interference in thereceived signal, while a downconverter 34 downconverts the filtered,received signal to an intermediate or baseband frequency signal, whichis then digitized into one or more digital streams. The receiver frontend 12 typically uses one or more mixing frequencies generated by thefrequency synthesizer 26.

The baseband processor 20 processes the digitized, received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. As such, the baseband processor 20 is generallyimplemented in one or more digital signal processors (DSPs).

On the transmit side, the baseband processor 20 receives digitized datafrom the control system 22, which it encodes for transmission. Theencoded data is output to the radio frequency transmitter section 14,where it is used by a modulator 36 to modulate a carrier signal that isat a desired transmit frequency. As described below in detail, theoutput of the modulator 36 is pre-distorted by pre-distortion circuitry38 to substantially cancel AM to AM and AM to PM distortion that occurswhen the modulated carrier signal is amplified by power amplifiercircuitry 40. The power amplifier circuitry 40 amplifies thepre-distorted modulated carrier signal to a level appropriate fortransmission from the antenna 16.

The power amplifier circuitry 40 provides gain for the signal to betransmitted under control of the power control circuitry 42, which ispreferably controlled by the control system 22. In essence, the powercontrol circuitry 42 operates to control a supply voltage provided tothe power amplifier circuitry 40 based on a transmit power controlsignal (TX POWER CONTROL) from the control system 22. The memory 24 maycontain software that allows many of these functions to be run.Alternatively, these may be a function of sequential logic structures asis well understood. In another embodiment, the supply voltage providedto the power amplifier circuitry 40 may be provided based on anamplitude component of a pre-distorted polar transmit signal from thepre-distortion circuitry 38. In yet another embodiment, the amplitudecomponent of the pre-distorted polar transmit signal may be provided tothe power control circuitry 42 and combined with the transmit powercontrol signal (TX POWER CONTROL) to control the supply voltage providedto the power amplifier circuitry 40.

A user may interact with the mobile terminal 10 via the interface 28,which may include interface circuitry 44 associated with a microphone46, a speaker 48, a keypad 50, and a display 52. The interface circuitry44 typically includes analog-to-digital converters, digital-to-analogconverters, amplifiers, and the like. Additionally, it may include avoice encoder/decoder, in which case it may communicate directly withthe baseband processor 20.

The microphone 46 will typically convert audio input, such as the user'svoice, into an electrical signal, which is then digitized and passeddirectly or indirectly to the baseband processor 20. Audio informationencoded in the received signal is recovered by the baseband processor20, and converted into an analog signal suitable for driving speaker 48by the interface circuitry 44. The keypad 50 and display 52 enable theuser to interact with the mobile terminal 10, input numbers to be dialedand address book information, or the like, as well as monitor callprogress information.

While the present invention is well-suited for incorporation into amobile terminal, such as the mobile terminal 10 just described, itshould be noted that the present invention is well-suited for use in anywireless transmitter. Types of wireless transmitters include thetransmitter section 14 of the mobile terminal 10, a wireless transmitterassociated with a wireless LAN, and the like. As such, the presentinvention is not limited to a particular apparatus.

Historically, as illustrated in FIG. 2, the modulator 36 directed itsoutput to the power amplifier circuitry 40. In an exemplary prior artembodiment, the modulator 36 included a rectangular-to-polar converter(not shown) and directed an amplitude signal (r) and a phase signal (φ)to the power amplifier circuitry 40. The amplitude signal (r) controlledthe power supply voltage of the power amplifier circuitry 40,potentially replacing or including the power control circuitry 42, whilethe phase signal (φ) was amplified by the power amplifier circuitry 40to create A_(DESIRED)∠φ_(DESIRED). The output (A_(O)∠_(O)) of the poweramplifier circuitry 40 was corrupted by AM to PM distortion within thenon-linear power amplifier circuitry 40, represented by φ(r), and AM toAM distortion, represented by A(r), resulting in an output signal ofA_(DESIRED)*A(r)∠φ_(DESIRED)+φ(r).

The pre-distortion circuitry 38 (FIG. 1) of the present inventioncorrects the AM to AM and AM to PM distortion within the power amplifiercircuitry 40 by preliminarily distorting the amplitude (r) and phase (φ)signals such that when they are amplified by the power amplifiercircuitry 40, an amplitude pre-distortion element cancels the AM to AMdistortion element A(r) and a phase pre-distortion element cancels theAM to PM distortion element φ(r).

Prior to discussing the details of the pre-distortion circuitry 38 ofthe present invention, it should be noted that in the past, polynomialpre-distortion was based on a polynomial for amplitude pre-distortionand a polynomial for phase pre-distortion. The two polynomials were usedto distort a signal prior to amplification by the power amplifiercircuitry 40 in order to offset the AM to AM and AM to PM distortion ofthe power amplifier circuitry 40. However, the two polynomials wereoften at least third order polynomials and are costly to implement.Further, the two polynomials may not effectively offset the AM to AM andAM to PM distortion of the power amplifier circuitry 40 at every powerlevel of the power amplifier circuitry 40.

FIG. 3A illustrates the pre-distortion circuitry 38 according to oneembodiment of the present invention. The pre-distortion circuitry 38includes rectangular-to-polar conversion circuitry 54, amplitudepre-distortion circuitry 56, phase pre-distortion circuitry 58,polynomial selection circuitry 60, memories 62 and 64, pre-distortioncoefficient storage circuitry 66, and polar-to-rectangular conversioncircuitry 68. The rectangular-to-polar conversion circuitry 58 mayoptionally be part of the modulator 36. Further, thepolar-to-rectangular conversion circuitry 58 is optional such that thepower amplifier circuitry 40 receives a pre-distorted polar signal fromthe pre-distortion circuitry 38.

In general, the pre-distortion circuitry 38 operates to essentiallycancel the AM to AM and AM to PM distortion of the power amplifiercircuitry 40. More specifically, the rectangular-to-polar conversioncircuitry 54 converts the modulated carrier signal from the modulator36, which is a rectangular signal having an in-phase (I) and aquadrature phase (Q) component, into a polar signal having an amplitudecomponent (r) and a phase component (φ). The amplitude component (r) isdirected to each of the amplitude pre-distortion circuitry 56, the phasepre-distortion circuitry 58, and the polynomial selection circuitry 60.

The amplitude pre-distortion circuitry 56 operates to generate anamplitude compensation signal A′(r) and provides a pre-distortedamplitude component (r′) of a pre-distorted polar signal. In oneembodiment, the amplitude pre-distortion circuitry 56 adds thecompensation signal A′(r) to the amplitude component (r) of the polarsignal. In another embodiment, the amplitude pre-distortion circuitry 56multiplies the compensation signal A′(r) and the amplitude component (r)of the polar signal. As described below in more detail, the compensationsignal A′(r) is such that it essentially cancels the AM to AM distortionA(r) of the power amplifier circuitry 40. The phase pre-distortioncircuitry 58 operates to generate a phase compensation signal φ′(r) and,in this embodiment, adds the compensation signal φ′(r) to the phasecomponent (φ) of the polar signal to provide a phase component (φ+φ′(r))of the pre-distorted polar signal. As described below in more detail,the compensation signal φ′(r) is essentially the additive inverse of theAM to PM distortion of the power amplifier circuitry 40.

The AM to AM distortion A(r) and the AM to PM distortion φ(r) changewhen a power level of the power amplifier circuitry 40 changes. Thus, inorder to more accurately cancel the AM to AM distortion A(r) and the AMto PM distortion φ(r), the pre-distortion circuitry 38 of the presentinvention generates the compensation signals A′(r) and φ′(r) such thatthe compensation signals A′(r) and φ′(r) are dependant upon the powerlevel of the power amplifier circuitry 40. Further, as commonly known,the AM to AM distortion A(r) and the AM to PM distortion φ(r) for thepower amplifier are non-linear over a range of input voltages.Accordingly, if the compensation signals A′(r) and φ′(r) are eachdescribed as a single polynomial for each power level, the polynomialfor each power level is typically third order or higher. Implementationof a third order polynomial is complex. However, if the range of inputvalues of the power amplifier circuitry 40 is divided into a number ofsubset, then each of the compensation signals A′(r) and φ′(r) for eachpower level can each be described by a number of low-order polynomialseach describing the compensation signal for one of the number of subsetsof the input value range.

Thus, according to the present invention, the compensation signal A′(r)is described by at least two groups of multiple low-order polynomials,where each of the multiple polynomials in each of the groups is definedby a set of coefficients. Each of the groups of multiple low-orderpolynomials corresponds to one of the power levels of the poweramplifier circuitry 40, and each of the multiple low-order polynomialsdescribes the compensation signal A′(r) for a subset of the input valuerange of the power amplifier circuitry 40. Similarly, the compensationsignal φ′(r) is described as at least two groups of multiple low-orderpolynomials, where each of the multiple polynomials in each of thegroups is defined by a set of coefficients. Each of the groups ofmultiple low-order polynomials corresponds to one of the power levels ofthe power amplifier circuitry 40, and each of the multiple low-orderpolynomials describes the compensation signal φ′(r) for one of thesubsets of the input range of the power amplifier circuitry 40.

In an exemplary embodiment, the power levels correspond to 2 dBm powersteps in the output power of the power amplifier circuitry 40. Thiscorresponds to the power steps defined in the EuropeanTelecommunications Standards Institute (ETSI) standards. Further, itshould be noted that the input value range may be divided into anynumber of subsets. The greater the number of subsets, the lower theorder to the multiple polynomials. If the number of subsets issufficiently large, each of the multiple polynomials may be a straightline. However, it may be desirable to have a low number of subsets, suchas two or three, in order to reduce the size of the memories 62 and 64needed to store the sets of coefficients and to reduce the complexity ofthe pre-distortion circuitry 38.

It should also be noted that, for each power level, the coefficients foreach polynomial for each subset of the input value range are determinedsuch that there are no discontinuities in the compensation signal A′(r)or φ′(r). More specifically, end-points of the polynomials for eachsubset of the input value range are constrained such that the end-pointsof the polynomials in adjacent subsets are aligned. This may beaccomplished in numerous ways. For example, the end-points of adjacentsubsets may be aligned by examining the end-points of the adjacentsubsets and offsetting each of the end-points to force alignment.

A set of coefficients for each polynomial in each group of multiplepolynomials for both of the compensation signals A′(r) and φ′(r) arestored by the pre-distortion coefficient storage circuitry 66. Thepre-distortion coefficient storage circuitry 66 may include software andhardware and may be programmable such that the coefficients can beprogrammed for a particular power amplifier circuitry 40 or such thatthe coefficients may be updated during operation. Based on the transmitpower control signal (TX POWER CONTROL), the pre-distortion coefficientstorage circuitry 66 provides coefficients for the group of multiplepolynomials corresponding to the current power level for thecompensation signal A′(r) to the memory 62 and provides coefficients forthe group of multiple polynomials corresponding to the current powerlevel for the compensation signal φ′(r) to the memory 64.

In operation, the amplitude component (r) of the polar signal isprovided to each of the amplitude pre-distortion circuitry 56, the phasepre-distortion circuitry 58, and the polynomial selection circuitry 60.Based on the amplitude component (r) of the polar signal, the polynomialselection circuitry 60 generates a selection signal that selects a setof coefficients from the group of coefficients stored in each of thememories 62 and 64 to be sent to the amplitude and phase pre-distortioncircuitries 56 and 58. The polynomial selection circuitry 60 generatesthe selection signal by determining the subset of the range of inputvalues in which the value of the amplitude component (r) falls. Based onthe selection signal, the memory 62 transfers the set of coefficientsdefining the polynomial describing the amplitude compensation signalA′(r) for the current power level and the current amplitude component(r) of the polar signal to the amplitude pre-distortion circuitry 56.Also based on the selection signal, the memory 64 transfers the set ofcoefficients defining the polynomial describing the phase compensationsignal φ′(r) for the current power level and the current amplitudecomponent (r) of the polar signal to the phase pre-distortion circuitry58.

Once the amplitude pre-distortion circuitry 56 receives the set ofcoefficients defining the polynomial describing the compensation signalA′(r) for the current power level and the current amplitude component(r) of the polar signal, the amplitude pre-distortion circuitry 56generates the amplitude compensation signal A′(r), which can bedescribed by the following equation:

${{A^{'\;}(r)} = {\sum\limits_{i = 0}^{N}\;{C_{A,i}( {r(t)} )}^{i}}},$where C_(A,i) are the coefficients from the memory 62. As an example, ifN=2, the equation expands to the following:A′(r)=C _(A,0) +C _(A,1) r(t)+C _(A,2)(r(t))²where r(t) is the amplitude component (r) from the rectangular-to-polarconverter 54. It is readily apparent that A′(r) has an offset term, alinear term, and a squared term. In an exemplary embodiment, the offsetterm C_(A,0) and the coefficient for the linear term C_(A,1) are zero.An offset term would act the same as increasing or decreasing the outputpower level. Since, in one embodiment, the power control circuitry 42already addresses this, it may not be necessary to repeat the controlhere. Likewise, a linear term would only change the fundamentalamplitude and not change the shape of the curve, so a linear term forthe amplitude compensation signal A′(r) may not be needed.

When the exemplary embodiment A′(r) is combined with the amplitudecomponent r(t) in the amplitude pre-distortion circuitry 56, thecombined signal is:r(t)=r(t)+A′(r).Thus, when C_(A,0)=C_(A,1)=0,r′(t)=r(t)+C _(A,2)(r(t))²which converts easily to the following:r′(t)=r(t)*[1+C _(A,2)(r(t))]Thus, the effect is to multiply the term r(t) by a correction factor,which in this example is 1+C_(A,2)(r(t)). This signal then passesthrough the power amplifier circuitry 40 with AM to AM distortion. Thisdistortion, as previously noted, is A(r). The goal is thus to make thecorrection factor the multiplicative inverse of the AM to AM distortionsuch that, in this example, A(r)*[1+C₂(r(t))]=1. When this condition istrue, the AM to AM distortion has been canceled.

Alternatively, if the amplitude pre-distortion circuitry 56 operated tomultiply the amplitude component (r) and the amplitude compensationsignal A′(r), then the compensation signal A′(r) could have a linearterm and an offset term. In this embodiment, the pre-distorted amplitudecomponent (r′) is equal to r(t)*A′(r). The goal in this embodiment is tomake the compensation signal A′(r) the multiplicative inverse of the AMto AM distortion such that A(r)*A′(r)=1 and the AM to AM distortion iscanceled.

Similarly, once the phase pre-distortion circuitry 56 receives thecoefficients of the polynomial describing the phase compensation signalφ′(r) for the current power level and the current amplitude component(r) of the polar signal, the phase pre-distortion circuitry 58 generatesthe phase compensation signal φ′(r), which can be described by thefollowing equation:

${{\phi^{'\;}(r)} = {\sum\limits_{i = 0}^{N}\;{C_{\phi,i}( {r(t)} )}^{i}}},$where C_(φ,i) are the coefficients from the memory 64. As an example,when N=2, the equation expands to the following:φ′(r)=C _(φ,0) +C _(φ,1) r(t)+C _(φ,2)(r(t))².It is readily apparent that φ′(r) of this example has an offset term, alinear term, and a quadratic term. When the phase pre-distortioncircuitry 58 combines the compensation signal φ′(r) with r(t), thecombined signal is the pre-distorted phase component (r(t)+φ′(r)). Thus,when this signal is converted to passes through the power amplifiercircuitry 40, the compensation signal φ′(r) substantially cancels the AMto PM distortion. This distortion, as previously noted, is φ(r). Thegoal is thus to make the φ′(r) equal to the additive inverse of the AMto PM distortion φ(r) such that φ(r)+φ′(r)=0. When this condition istrue, the AM to PM distortion has been canceled.

The pre-distorted amplitude component (r′) and the pre-distorted phasecomponent (φ+φ′(r)) form a pre-distorted polar transmit signal. In thisembodiment, the pre-distorted polar signal is provided to thepolar-to-rectangular conversion circuitry 68 where it is converted to apre-distorted rectangular signal having an in-phase (I) and a quadraturephase (Q) component. The pre-distorted rectangular signal is provided tothe power amplifier circuitry 40 for amplification. When thepre-distorted rectangular signal is provided to the power amplifiercircuitry, the AM to AM distortion and the AM to PM distortion of thepower amplifier circuitry 40 is essentially canceled, and the desiredoutput signal A_(DESIRED)∠φ_(DESIRED) is generated.

FIG. 3B illustrates the pre-distortion circuitry 38 according to anotherembodiment of the present invention. This embodiment of thepre-distortion circuitry operates essentially the same as thepre-distortion circuitry 38 of FIG. 3A. However, in this embodiment, thepre-distorted amplitude component (r′) and the pre-distorted phasecomponent (φ+φ′(r)) output by the amplitude pre-distortion circuitry 56and the phase pre-distortion circuitry 58, respectively, are notconverted to a rectangular signal by the polar-to-rectangular conversioncircuitry 68 (FIG. 3A). Instead, the pre-distorted amplitude component(r′) and the pre-distorted phase component (φ+φ′(r)) form apre-distorted polar signal that is provided to the power amplifiercircuitry 40. In this embodiment, the power amplifier circuitry 40 is apolar modulation power amplifier and the amplitude component of thepre-distorted polar signal controls the output power of the poweramplification circuitry 40, thereby modulating the phase component ofthe pre-distorted polar signal. However, the effect of thepre-distortion circuitry 38 is to essentially cancel the AM to AM and AMto PM distortion of the power amplifier circuitry 40. In anotherembodiment, the pre-distorted amplitude component (r′) may be combinedwith the transmit power control signal (TX POWER CONTROL), and thecombination provided to the power control circuitry 42 to control thesupply voltage provided to the power amplifier circuitry 40.

The present invention provides substantial opportunity for variationwithout departing from the spirit or scope of the present invention. Forexample, although the pre-distortion circuitry 38 is described as havingboth the amplitude pre-distortion circuitry 56 and the phasepre-distortion circuitry 58, the pre-distortion circuitry 38 may haveonly the amplitude pre-distortion circuitry 56 or only the phasepre-distortion circuitry 58.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present invention. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

1. A wireless transmitter comprising: power amplifier circuitry havingan AM to AM distortion characteristic for an input value range andadapted to amplify a pre-distorted transmit signal prior totransmission; and pre-distortion circuitry comprising amplitudepre-distortion circuitry adapted to: distort an amplitude component of apolar transmit signal based on a set of coefficients defining apolynomial describing an amplitude compensation signal that essentiallycancels the AM to AM distortion of the power amplifier circuitry,thereby providing a pre-distorted amplitude component of thepre-distorted transmit signal; select the set of coefficients from aplurality of sets of coefficients defining a corresponding plurality ofpolynomials describing the amplitude compensation signal for each of atleast two subsets of the input value range for each of at least twopower levels of the power amplifier circuitry; and add the amplitudecomponent of the polar transmit signal and the amplitude compensationsignal, thereby distorting the amplitude component to provide thepre-distorted amplitude component of the pre-distorted transmit signal,such that the sum of the amplitude component and the amplitudecompensation signal is essentially equivalent to a product of theamplitude component and a correction factor wherein the correctionfactor is essentially a multiplicative inverse of the AM to AMdistortion of the power amplifier circuitry.
 2. The wireless transmitterof claim 1 wherein the amplitude pre-distortion circuitry is furtheradapted to: select a group of the plurality of sets of coefficients thatdefine multiple polynomials describing the amplitude compensation signalfor one of the at least two power levels based on a power controlsignal; select the set of coefficients from the group of the pluralityof sets of coefficients based on the amplitude component of the polartransmit signal; and generate the amplitude compensation signal based onthe set of coefficients.
 3. The wireless transmitter of claim 1 whereinthe pre-distortion circuitry further comprises phase pre-distortioncircuitry adapted to: distort a phase component of the polar transmitsignal based on a second set of coefficients defining a polynomialdescribing a phase compensation signal that essentially cancels an AM toPM distortion of the power amplifier circuitry for the input valuerange, thereby providing a pre-distorted phase component of thepre-distorted transmit signal; and the second set of coefficientsselected from a second plurality of sets of coefficients defining acorresponding second plurality of polynomials describing the phasecompensation signal for each of the at least two subsets of the inputvalue range for each of the at least two power levels of the poweramplifier circuitry.
 4. The wireless transmitter of claim 3 wherein thephase pre-distortion circuitry is further adapted to: select a group ofthe second plurality of sets of coefficients that define multiplepolynomials describing the phase compensation signal for one of the atleast two power levels based on a power control signal; select thesecond set of coefficients from the group of the second plurality ofsets of coefficients based on the amplitude component of the polartransmit signal; generate the phase compensation signal based on thesecond set of coefficients; and combine the phase component of the polartransmit signal and the phase compensation signal, thereby distortingthe phase component to provide the pre-distorted phase component of thepre-distorted transmit signal.
 5. The wireless transmitter of claim 4wherein the phase pre-distortion circuitry is further adapted to combinethe phase component of the polar transmit signal and the phasecompensation signal by adding the phase component of the polar transmitsignal and the phase compensation signal wherein the second plurality ofsets of coefficients define the phase compensation signal such that thephase compensation signal is essentially the additive inverse of the AMto PM distortion of the power amplifier circuitry.
 6. The wirelesstransmitter of claim 1 wherein the plurality of sets of coefficients areprogrammable.
 7. The wireless transmitter of claim 1 wherein thepre-distortion circuitry further comprises rectangular-to-polarconversion circuitry adapted to provide the polar transmit signal byconverting a rectangular transmit signal into the polar transmit signal.8. The wireless transmitter of claim 3 wherein the pre-distortioncircuitry further comprises polar-to-rectangular conversion circuitryadapted to convert the pre-distorted amplitude components and thepre-distorted phase components into a rectangular pre-distorted signaland provide the rectangular pre-distorted signal to the power amplifiercircuitry as the pre-distorted transmit signal.
 9. The wirelesstransmitter of claim 3 wherein the pre-distorted transmit signal is apolar signal comprising the pre-distorted amplitude and phasecomponents.
 10. A wireless transmitter comprising: power amplifiercircuitry having an AM to PM distortion characteristic for an inputvalue range and adapted to amplify a pre-distorted transmit signal priorto transmission; and pre-distortion circuitry comprising phasepre-distortion circuitry adapted to: distort a phase component of apolar transmit signal based on a set of coefficients defining apolynomial describing a phase compensation signal that essentiallycancels the AM to PM distortion of the power amplifier circuitry,thereby providing a pre-distorted phase component of the pre-distortedtransmit signal; select the set of coefficients from a plurality of setsof coefficients defining a corresponding plurality of polynomialsdescribing the phase compensation signal for each of at least twosubsets of the input value range for each of at least two power levelsof the power amplifier circuitry; and add the phase component of thepolar transmit signal and the phase compensation signal, therebydistorting the phase component to provide the pre-distorted phasecomponent of the pre-distorted transmit signal wherein the plurality ofsets of coefficients define the phase compensation signal such that thephase compensation signal is essentially the additive inverse of the AMto PM distortion of the power amplifier circuitry.
 11. The wirelesstransmitter of claim 10 wherein the phase pre-distortion circuitry isfurther adapted to: select a group of the plurality of sets ofcoefficients that define multiple polynomials describing the phasecompensation signal for one of the at least two power levels based on apower control signal; select the set of coefficients from the group ofthe plurality of sets of coefficients based on an amplitude component ofthe polar transmit signal; and generate the phase compensation signalbased on the set of coefficients.
 12. The wireless transmitter of claim10 wherein the pre-distortion circuitry further comprises amplitudepre-distortion circuitry adapted to: distort an amplitude component ofthe polar transmit signal based on a second set of coefficients defininga polynomial describing a amplitude compensation signal that essentiallycancels an AM to AM distortion of the power amplifier circuitry for theinput value range, thereby providing a pre-distorted amplitude componentof the pre-distorted transmit signal; and the second set of coefficientsselected from a second plurality of sets of coefficients defining acorresponding second plurality of polynomials describing the amplitudecompensation signal for each of the at least two subsets of the inputvalue range for each of the at least two power levels of the poweramplifier circuitry.
 13. The wireless transmitter of claim 12 whereinthe amplitude pre-distortion circuitry is further adapted to: select agroup of the second plurality of sets of coefficients that definemultiple polynomials describing the amplitude compensation signal forone of the at least two power levels based on a power control signal;select the second set of coefficients from the group of the secondplurality of sets of coefficients based on the amplitude component ofthe polar transmit signal; generate the amplitude compensation signalbased on the second set of coefficients; and combine the amplitudecomponent of the polar transmit signal and the amplitude compensationsignal, thereby distorting the amplitude component to provide thepre-distorted amplitude component of the pre-distorted transmit signal.14. The wireless transmitter of claim 13 wherein the amplitudepre-distortion circuitry is further adapted to combine the amplitudecomponent of the polar transmit signal and the amplitude compensationsignal by multiplying the amplitude component of the polar transmitsignal and the amplitude compensation signal and wherein the secondplurality of sets of coefficients define the amplitude compensationsignal such that the amplitude compensation signal is essentially amultiplicative inverse of the AM to AM distortion of the power amplifiercircuitry.
 15. The wireless transmitter of claim 13 wherein theamplitude pre-distortion circuitry is further adapted to combine theamplitude component of the polar transmit signal and the amplitudecompensation signal by adding the amplitude component of the polartransmit signal and the amplitude compensation signal.
 16. The wirelesstransmitter of claim 15 wherein the second plurality of sets ofcoefficients define the amplitude compensation signal such that the sumof the amplitude component and the amplitude compensation signal isessentially equivalent to a product of the amplitude component and acorrection factor wherein the correction factor is essentially themultiplicative inverse of the AM to AM distortion of the power amplifiercircuitry.
 17. The wireless transmitter of claim 12 wherein thepre-distortion circuitry further comprises polar-to-rectangularconversion circuitry adapted to convert the pre-distorted amplitudecomponent and the pre-distorted phase components into a rectangularpre-distorted signal and provide the rectangular pre-distorted signal tothe power amplifier circuitry as the pre-distorted transmit signal. 18.The wireless transmitter of claim 12 wherein the pre-distorted transmitsignal is a polar signal comprising the pre-distorted amplitude andphase components.
 19. The wireless transmitter of claim 10 wherein theplurality of sets of coefficients are programmable.
 20. The wirelesstransmitter of claim 10 wherein the pre-distortion circuitry furthercomprises rectangular-to-polar conversion circuitry adapted to providethe polar transmit signal by converting a rectangular transmit signalinto the polar transmit signal.